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Journal of Physics B Atomic Molecular and Optical Physics

Electron-polarized photon coincidence measurements on Ne*(3p) states

A W Baerveldt, J. van Eck, H G M Heideman

Published May 14, 1994• 6 citations
Publisher

Abstract

In an electron-polarized photon coincidence experiment the Stokes' parameters P 1 , P 2 , P 3 and P 4 have been determined for four Ne * (3p) to Ne * (3s) transitions at a fixed electron energy of 50 eV and in the electron angular scattering range 30 degrees -70 degrees . The results indicate a tendency towards positive values for P 3 at the lowest scattering angles studied and a strong tendency towards zero and even negative values for P 4 . In all cases a drop in the total polarization degree between 30 degrees and 50 degrees electron scattering angle is observed, indicating that there is a decrease of coherence at these scattering angles.

DOI
10.1088/0953-4075/27/9/021

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In an electron-polarized photon coincidence experiment the Stokes' parameters P 1 , P 2 , P 3 and P 4 have been determined for four Ne * (3p) to Ne * (3s) transitions at a fixed electron energy of 50 eV and in the electron angular scattering range 30 degrees -70 degrees . The results indicate a tendency towards positive values for P 3 at the lowest scattering angles studied and a strong tendency towards zero and even negative values for P 4 . In all cases a drop in the total polarization degree between 30 degrees and 50 degrees electron scattering angle is observed, indicating that there is a decrease of coherence at these scattering angles.
The delayed coincidence technique was applied to investigate angular and polarization correlations between electrons scattered inelastically from helium atoms and photons emitted in the decay of n1P states. \n \nElectron photon angular correlations were studied for the excitation of the 21P and 31P states of helium at an incident electron energy of 50 eV and for electron scattering angles in the range of 20° - 102° and 35° - 55o respectively. From the analysis of these measurements the excitation parameters λ = (|a¬o| 2)/(2| a1| 2+|ao| 2) = σo/σ and the magnitude |x| of the phase difference between the excitation amplitudes a_ML for the M_L = 1 and M_L = 0 states are obtained. \n \nUsing the 3¬¬1P - 21S decay light the electron-photon polarization correlation were measured for the 31P state of helium at electron impact energies 50, 80, 120 and 160 eV and for electron scattering angles between 27.5° and 108°. The Stokes parameters n1, n2 and n3 were derived and the sign change of the angular momentum transfer < Ly > to the atom was investigated using the circular polarization results (n2). A sign change was confirmed for all energies studied and the scattering angle of the sign change was found to increase with decreasing electron energy. The excitation parameters λ and x have been derived and the present results are compared with theoretical calculations and with previous experimental values obtained from angular and polarization correlation measurements.
An electron-photon coincidence technique has been used to study the coherence of the excitation of the 3 2 P state in sodium. The total polarisation of the decay photons has been determined by measuring the linear and circular components of the Stokes's vector. Analysis of the reduced degree of polarisation mod P mod shows that the excitation is coherent for electrons which have been scattered through the angular range from 3 to 10 degrees . The concept of reduced polarisation assumes that the Percival-Seaton hypothesis applies to the collision. Thus the authors deduce that the nuclear spin plays no role in the excitation process at this energy and over these scattering angles. Scattering parameters have been deduced from the individual components of the Stokes's vector. These have been compared with those predicted by calculations based on the first Born and distorted-wave polarised orbital approximations. It is found that neither theory adequately describes the interaction.
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The authors present circular-polarisation measurements of 3 1 P to 2 1 S photons (501.6 nm) detected in delayed coincidence with electrons inelastically scattered upon excitation of the 3 1 P state of helium. They performed these experiments as a function of the electron scattering angle at incident electron energies of 80, 60 and 50 eV in order to study the orbital angular momentum transfer during the collision. The experimental results are compared with previous angular correlation data and with theory. For each incident electron energy they found a sign reversal of the angular momentum transfer as a function of the electron scattering angle.
For neon 3p states excited by transversely polarized electrons, the integrated Stokes parameters, and , have been measured from threshold to 300 eV for the decay photons of 692.9, 667.8 and 717.4 nm wavelengths from the and states, respectively. The and parameters for the 692.9 and 667.8 nm transitions indicate similar excitation mechanisms consistent with their similar ` L S component states'. The non-zero indicates the breakdown of L S -coupling and the large values near threshold suggest a significant exchange process. For the 717.4 nm transition, the three Stokes parameters are much smaller than those of the other two lines. The low is surprising since the large (75.4%) -coupled component state was expected to cause large exchange effects.